A typical CMOS (complementary metal-oxide semiconductor) image sensor has a pixel array and a readout circuit. Each pixel circuit of the pixel array includes a photodiode and an amplifier. The voltage generated in the photodiode is amplified by the amplifier and output to a column signal line. A pixel is read out twice by the readout part in order to perform a noise cancellation treatment known as CDS (correlated double sampling). Pixel signals of dark level (N) and signal level (NS) are read out, and a new pixel signal (S) corresponding to the difference between the signals (N-NS) is obtained. Analog-digital conversion (AD conversion) is usually performed to the pixel signal (S) after CDS in order to facilitate image processing.
The signal charge of each pixel is read out by a method that combines a parallel method and a sequential method. Each row of the pixel array is selected sequentially. Pixel signals are read out in parallel from the plural columns of the selected row. The degree of parallelism varies corresponding to the degree of readout processing performed for each column. Table 1 shows the content of readout processing performed for each column and the degree of parallelism.
TABLE 1Degree ofN/NSSignalADparallelismmemoryCDSamplificationconversionLow◯———Medium◯◯◯—High◯◯◯◯
Increasing the degree of parallelism can help improve the performance while lowering power consumption. This is the reason that integration of elements at the column level has been energetically promoted in the case of improving the resolution of the image sensor. In recent years, a CMOS image sensor equipped with an AD converter in each column has been developed.